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Roland B. Stull and Edwin W. Eloranta

Interactions between fair-weather cumulus clouds and mixed-layer thermals were the focus of a one-month field experiment in Oklahoma. This experiment, called Boundary Layer Experiment—1983 (BLX83), combined remote sensors, surface observations, balloon platforms, and aircraft measurements to study the kinematics at the top of the daytime convective boundary layer. Emphasis was placed on the study of the entrainment zone, and on the relationship between individual thermals as identified by lidar and turbulent motions and fluxes as measured by aircraft and sodar.

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Shane D. Mayor and Edwin W. Eloranta

Abstract

Spatially resolved wind fields are derived by cross correlation of aerosol backscatter data from horizontal and vertical scans of the University of Wisconsin volume imaging lidar during the 1997/98 Lake-Induced Convection Experiment. Data from three cases are analyzed. The first two cases occurred on 10 and 13 January 1998 during cold-air outbreaks. Horizontal scans at 5 m above the lake reveal cellular structure of the steam fog. Vector winds are derived with 250-m spatial resolution over 60 and 36 km2 areas. These wind fields show acceleration and veering of offshore flow in the convective internal boundary layer along the upwind edge of Lake Michigan. The wind fields are used to compute divergence and vorticity. Effects of shoreline shape and topography are evident in the data. Horizontal wind speeds derived from vertical scans show the effects of convection on the vertical distribution of momentum. In the third case, 21 December 1997, a well-defined, shallow density current flowing offshore at ≈1 m s−1 is observed in the presence of larger-scale (3–4 m s−1) onshore flow. Winds on both sides of the land-breeze boundary as well as the three-dimensional structure of the event were recorded and analyzed.

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William P. Hooper and Edwin W. Eloranta

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During the Central Illinois Rainfall Chemistry Experiment (CIRCE), the University of Wisconsin lidar measured wind and turbulence profiles through the planetary boundary layer for a 32-h period in conjunction with surface observations, radiosonde soundings and kytoon profiles made by Argonne National Laboratory. The lidar profiles were made using an advection model for aerosol inhomogeneities as described by Sroga et al. We discuss improvements to this model and explore the accuracy of the lidar wind and boundary layer depth measurements. In addition, the temporal variation of lidar data was utilized to measure boundary layer depth objectively. Cross sections of the speed, direction and rms variation of the wind for the 32-h period show the daytime convective layer, nocturnal stable layer and transitional periods.

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Shane D. Mayor, Gregory J. Tripoli, and Edwin W. Eloranta

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The authors apply data analysis techniques that demonstrate the power of using volume imaging lidar observations to evaluate several aspects of large-eddy simulations (LESs). They present observations and simulations of an intense and spatially evolving convective boundary layer on 13 January 1998 during the Lake-Induced Convection Experiment (Lake-ICE). To enable comparison of observed and simulated eddy structure, aerosol scattering was estimated from LES output of relative humidity, a passive tracer, and liquid water. Spatial and temporal correlation functions of aerosol structure from horizontal planes reveal the mean and turbulent convective structure. The correlation functions of the observed and simulated aerosol backscatter are presented as a function of altitude and offshore distance. Best-fit ellipses to the closed contours encircling the origin of the correlation functions are used to obtain the mean ellipticity and orientation of the structures. The authors demonstrate that these two parameters are not sensitive to minor changes in the functional relationship between humidity and optical scattering. The lidar-derived mean wind field is used as a reference for evaluating the LES mean flow.

The ellipses from lidar data indicate that structures near the surface tend to be aligned with the mean wind direction, while in the entrainment zone they are aligned perpendicular to the mean wind direction. In the middle of the mixed layer, convective plumes tended to be circular and, therefore, had no preferred orientation at small lags. At longer lags, however, the correlation functions from the middle of the mixed layer show that the observed convective plumes were organized into linear bands oriented perpendicular to the mean wind direction. The perpendicular bands suggest the important role of gravity waves in organizing convective structures. The study shows that the model generates reasonable coherent structures (open cells) where the LES technique is expected to perform poorly (near the surface) and fails to capture the wind-perpendicular organization of closed cells in the middle of the mixed layer where the LES technique is expected to be robust. The authors attribute this failure to the boundary conditions that limited the growth of waves above the mixed layer.

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Jeffery T. Sroga, Edwin W. Eloranta, and Ted Barber

Abstract

A lidar technique for measuring wind in the atmospheric boundary is presented. Inhomogeneities in ambient aerosol content are used as tracers of the wind. This technique yields both horizontal components of the wind and the wind velocity variance. These results are achieved using a model which assumes an isotropic Gaussian distribution of turbulent velocities. Experimental results comparing lidar wind measurements with winds derived from radar-tracked pilot balloons and tower-mounted anemometers show good agreement. Wind measurements have been obtained at slant range distances up to 6.5 km.

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Roni Avissar, Edwin W. Eloranta, Kemal Gürer, and Gregory J. Tripoli

Abstract

A large-eddy simulation (LES) model was used to simulate the convective boundary layer (CBL) that developed on 1 July 1987, over the domain of the First International Satellite Land Surface Climatology Project Field Experiment (FIFE). Three simulations were produced using different boundary conditions at the ground surface, namely, (i) spatial distribution of topography and spatial distribution of surface heat fluxes; (ii) spatial distribution of topography but mean surface heat fluxes; and (iii) no topography and mean surface heat fluxes. The diurnal variation of mean surface fluxes and their spatial distribution were derived from the FIFE network of observations. In all cases, the model was initialized with the atmospheric sounding observed in this domain at 0700, and run until 1500 local time. The resulting mean profiles of temperature and specific humidity were compared to those observed with atmospheric soundings at 0900, 1030, and 1230 local time. The simulated structure of turbulence was qualitatively compared with that obtained from a volume-imaging lidar (VIL) scanning the CBL over the simulated domain during that day. Power spectra and autocorrelations of mixing ratio were calculated from the model outputs and were compared to those obtained from the VIL.

Overall, the model performed quite well. Observed atmospheric soundings were within 1 K and 1 g kg−1 of the simulated mean profiles of temperature and specific humidity, respectively, and indicated that the model correctly predicts the CBL height. Similarities in the structure of the eddies obtained from the model and the VIL were clearly identified. Spectral analysis indicated that resolved eddies (i.e., eddies larger than 200 m) are relatively well simulated with the model, but that the energy cascade is not well represented by the Deardorff 1.5-order-of-closure subgrid-scale parameterization. Autocorrelation analysis indicated that the model correctly simulates the characteristic size of the eddies, but that their mean lifetime is longer than that observed with the VIL, indicating a too weak dissipation of the eddies by the subgrid-scale scheme. Thus, this study emphasized the need to develop better subgrid-scale parameterizations for LES models. The different simulations also indicated that topographical features of the order of 100 m and microβ-scale heterogeneity of surface heat fluxes had only a minor to modest impact on the CBL developing over a relatively humid surface.

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Timothy D. Crum, Roland B. Stull, and Edwin W. Eloranta

Abstract

Coincident observations of the daytime convective boundary layer over Oklahoma were made with the NCAR Queen Air aircraft and the University of Wisconsin ground-based lidar. The two data sets have been merged to provide a unique visual representation of the temperature, moisture, vertical velocity, turbulent kinetic energy and the momentum fluxes in a field of thermals. These data show that horizontal moisture profiles observed in thermals penetrating the entrainment zone tend to exhibit more of a top-hat profile than the corresponding temperature or vertical velocity profiles. The specific humidities observed at various heights including cloud base 1) are frequently nearly constant along the horizontal tracks within each thermal; 2) show thermal-to-thermal variability; and 3) have values nearly the same as found in the surface layer. This paper also proposes the concept of an “intromission zone” describing the zone of lateral entrainment at the edges of active thermals. For the data studied here, a lateral entrainment velocity of 0.3 m s−1 was observed.

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Gijs de Boer, Edwin W. Eloranta, and Matthew D. Shupe

Abstract

Macro- and microphysical properties of single-layer stratiform mixed-phase clouds are derived from multiple years of lidar, radar, and radiosonde observations. Measurements were made as part of the Mixed-Phase Arctic Clouds Experiment (MPACE) and the Study of Environmental Arctic Change (SEARCH) in Barrow, Alaska, and Eureka, Nunavut, Canada, respectively. Single-layer mixed-phase clouds occurred between 4% and 26% of the total time observed, varying with season and location. They had mean cloud-base heights between ∼700 and 2100 m and thicknesses between ∼200 and 700 m. Seasonal mean cloud optical depths ranged from 2.2 up. The clouds existed at temperatures of ∼242–271 K and occurred under different wind conditions, depending on season. Utilizing retrievals from a combination of lidar, radar, and microwave radiometer, mean cloud microphysical properties were derived, with mean liquid effective diameters estimated from 16 to 49 μm, mean liquid number densities on the order of 104–105 L−1, and mean water contents estimated between 0.07 and 0.28 g m−3. Ice precipitation was shown to have mean ice effective diameters of 50–125 μm, mean ice number densities on the order of 10 L−1, and mean water contents estimated between 0.012 and 0.031 g m−3. Mean cloud liquid water paths ranged from 25 to 100 g m−2. All results are compared to previous studies, and potential retrieval errors are discussed. Additionally, seasonal variation in macro- and microphysical properties was highlighted. Finally, fraction of liquid water to ice mass was shown to decrease with decreasing temperature.

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Edwin W. Eloranta, Roland B. Stull, and Elizabeth E. Ebert

Abstract

A calibration device was designed to fit over the Lyman-α (LA) probes on the NCAR King Air aircraft to allow the introduction of pure nitrogen, oxygen, and carbon dioxide gases into the probe's radiation path. With these three gases, it was possible to calculate three of the most important terms in the LA humidity equation: path length, reference voltage (radiation) and oxygen absorption. This calibration device was tested in France during the HAPEX-MOBILHY field program, and was found to perform successfully.

As a result of the calibration, it was found that the effective LA path lengths during HAPEX were significantly different from the “nominal” path length physically set at the start of the experiment. Also, the oxygen absorption cross section was over twice as large as the published values, suggesting that the emission spectra of the lamps used in the LA probes are contaminated with other emission lines. The measured LA probe output reference voltages for no absorption were found to be slowly varying in time, suggesting that inflight “floating” calibrations against another reference hygrometer are necessary, in addition to the pre- and post-flight calibrations on the ground using the test device.

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Nicholas P. Wilde, Roland B. Stull, and Edwin W. Eloranta

Abstract

Variations of the lifting condensation level (LCL) of surface layer air are documented based on data from the BLX83 field experiment in Oklahoma. For example, within a 25 km long region near Chickasha, the local LCL height was found to vary by 15–30% of its average height. This zone of variation, centered on the mean LCL height, is identified as the “LCL zone”. It is analogous to the entrainment zone for the local mixed layer depth. Cumulus clouds first form when the top of the entrainment zone reaches the bottom of the LCL zone. As more of the entrainment zone overlaps and reaches above the LCL zone, the cloud cover increases. Two case studies are presented to demonstrate the diagnosis of cumulus onset time and cloud cover amount using this overlap technique. Combined radar, aircraft, rawinsonde, and surface observations indicate that some of the air observed at cloud base has the same low LCL as that of the mean surface layer air. This leads us to speculate that some surface layer air is rising up to cloud base with relatively little dilution, perhaps within the cores of thermals.

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